The present invention relates to an electrophotographic copying machine and, more particularly, to a developing device for visualizing an electrostatic latent image as a toner image in an electrophotographic copying machine.
An electrophotographic copying machine produces an electrostatic latent image on a photoreceptor with an optical system. The latent image corresponds to an image on a copy document such as a manuscript or book to be copied. A developing device is provided so that toner particles are electrically adhered to the latent image, wherein the latent image becomes visible as a toner image. The toner image is transferred onto a copy paper via a transference charger.
The conventional developer is a two-component developer comprising toner particles and magnetic particles called carriers, so that the developer is magnetically attracted by a magnet roller to form a magnetic brush in contact with the photoreceptor, which is called a magnetic brush developing system. Of course, it is most preferable that only the toner particles are transferred to the electrostatic latent image on the photoreceptor. However, the carriers may inadvertently be attached to image portions and non-image portions on the photoreceptor, thereby resulting in degradation of the copied image and damages in controlling the density of the developer. Accordingly, it is expected that the carriers are prevented from being attached to the photoreceptor.
Conventionally, to prevent the attachment of the carriers onto the photoreceptor, the following are controlled: voltage difference between the charging voltage of the photoreceptor and the developing bias voltage, the separation between the magnet roller and an element for controlling attachment of the developer onto the magnet roller, the separation between the magnet roller and the photoreceptor, charging amounts of change associated with the carriers, and relative limitations of the shapes and the sizes of the carriers.
However, the above-mentioned methods for preventing the attachment of the carriers onto the photoreceptor require narrow design conditions. For example, controlling the separation between the photoreceptor and the magnet roller, and the selection of the developing bias voltage etc. are extremely complex.